After the Calvin Klein Jeans-hosted Cinema Society screening of "21" last night, guests headed over to the Mercer Hotel, where the sub-basement had been transformed into a mini casino, complete with blackjack tables. Stars Kate Bosworth and Jim Sturgess, who play math whizzes turned Vegas high rollers in the film, were there, but didn't indulge. Nor did Calvin Klein campaign boy Jamie Burke, for whom the stakes weren't quite high enough. "I threw down a twenty and the guy just looked at me," Burke said. "Apparently they're playing for fun." And when Burke was told that instead of cash the prize was cK boxer shorts, he reminded himself he didn't need to run the risk. "I can probably get those on my own." But that brought up an interesting point for Sturgess. "Wait, so if you're playing for your underwear, what happens when you lose? Instead of losing your shirt, do you lose your trousers?" he asked. "That'll make it easier to spot the losers. Just look for the guys without underwear on."